Vessel Stability and Naval Architecture Flashcards

The free surface effect reduces vessel stability because:

Answer: Liquid in a partially filled tank shifts to the low side when the vessel heels, raising the effective center of gravity

When a partially filled tank's liquid shifts to the low side as the vessel heels, the transverse shift of that weight raises the effective G, reducing GM and stability; this loss is quantified as the free surface correction.

The Plimsoll mark (load line) on a vessel indicates:

Answer: The maximum permissible loading depths for various seasonal and geographic zones

The load line marks the maximum depths to which a vessel may be loaded under different sea conditions and seasons (tropical, summer, winter, freshwater, etc.) to ensure adequate freeboard and safety.

In a vessel's stability booklet, the 'light ship' condition is defined as:

Answer: The completed vessel with no cargo, fuel, ballast, fresh water, stores, crew, or passengers

Light ship displacement is the weight of the vessel as built and equipped, with all machinery operational but carrying no cargo, fuel, lubricating oil, ballast, fresh water, stores, crew, or passengers.

In naval architecture, 'trim' refers to:

Answer: The difference between the forward draft and the after draft

Trim is the longitudinal inclination of a vessel, defined as the numerical difference between the forward and aft drafts; a vessel trimmed by the stern has a greater aft draft than forward draft.

IMO intact stability criteria for most vessels primarily require:

Answer: Sufficient area under the GZ curve across defined heel angle ranges to resist dynamic forces

IMO criteria (IS Code 2008) specify minimum areas under the GZ righting arm curve between defined angles of heel, ensuring sufficient dynamic stability energy to survive waves, wind, and cargo shift.

When calculating stability, an increase in the vessel's KG (height of center of gravity above keel) will:

Answer: Reduce stability by decreasing GM, bringing G closer to or above M

Since GM = KM − KG, any increase in KG directly reduces GM; if KG rises above KM the vessel becomes unstable with negative GM and may develop an angle of loll or capsize.

A 'stiff' vessel in terms of stability is one that:

Answer: Has a large GM, rolls quickly with a short period, and can be uncomfortable or dangerous for cargo

A stiff vessel has a large GM producing a strong righting moment, which results in a rapid, short-period snap roll that can be uncomfortable for crew, damaging to cargo, and stressful to the hull structure.